The present invention relates to a method for calibrating temperature-measuring resistors on a glass, glass-ceramic or similar substrate, especially conductor strip resistors on a glass-ceramic cooking surface.
To improve the usability of cooking apparatus with a glass-ceramic cooking surface it is desirable to measure and indicate the temperature in the cooking zone or to control the cooking process using the temperature measurement. For example, conductor strips can be applied, for example, to the underside of the cooking surface in the vicinity of the cooking zone and utilize temperature-dependent resistors (temperature-measuring resistors) of conductive material. The method is useable for either an electrically heated or gas-heated cooking apparatus as well as, among others, a grill unit. Noble metals, such as gold or platinum, are used as the conductor strip material preferably at high operating temperatures. Also silver may be used with lower maximum temperatures (under 400.degree. C.). This type of temperature measurement is described, for example, in German Patent Application DE 40 22 845 C2.
In general electric resistance increases approximately linearly with temperature in commonly used conductor strip materials and can be analyzed satisfactorily in the entire temperature range of interest with electronic means.
However it is disadvantageous that temperature-measuring resistors of the above-described type are subjected to aging in practical usage. Because of that the temperature-resistance characteristic curves of the temperature-measuring resistors can change to a considerable extent.
Typically the resistance of the temperature-measuring resistor changes by aging from 14 to 15 .OMEGA. at a temperature of 20.degree. C. (R.sub.20) at a standard operating temperature of about 600.degree. C. during a standard operating time of about 2000 h. For example, the resistance of 14 .OMEGA. which previously corresponded to 20.degree. C. is already measured at -8.degree. C. because of the change of R.sub.20 from 14 to 15 .OMEGA.. The resistance of 15.01 .OMEGA. which corresponded to a temperature of 50.degree. C. previously is already measured at 20.3.degree. C. and the resistance of 26.77 .OMEGA. which previously corresponded to a temperature of 400.degree. C. is already reached at 347.degree. C. At any rate, this corresponds to a deviation from a true reading of 29.7 to 53.degree.. Also manufacturer's tolerances in the making of the conductor strip resistors lead to variations in the above-mentioned variable size.